This invention relates to a servo track configuration for magnetic disk apparatus.
In recent years, a small sized magnetic disk apparatus is arranged such that data sectors for storage of data signals and servo sectors for storage of servo signals co-exist on the magnetic disk surface for effective utilization of the same without using a servo data surface for exclusive use in the positioning of a magnetic head as in the case of a large sized magnetic disk apparatus. The data sectors and servo sectors are alternately disposed on the magnetic disk surface provided with a plurality of tracks. If, in a state wherein the magnetic head is radially movably is close proximity to the magnetic disk surface, the magnetic disk is rotated, the magnetic head produces a signal obtained from the servo signal or data signal written on the track corresponding to the position at which the magnetic head is located. Since the signals from the servo sectors are intermittently produced from the magnetic head, the servo sectors are provided thirty or more in number for obtaining stable and detailed information as to the position of the magnetic head.
When it is desired to move the magnetic head onto a predetermined track, the difference between the track on which the magnetic head is currently located and a predetermined track onto which the magnetic head is to be moved (that is, the number of tracks to be passed over by the magnetic head) is supplied to a suitable servo device. When the number of tracks to be passed is not "0", the magnetic head is caused to move. Each time the magnetic head goes across a track, one is subtracted from the number of tracks to be passed over. When this track number has become "0", the moving operation for the magnetic head is stopped. In order to detect whether or not the magnetic head has gone across a track, use is made of an output signal obtained from the magnetic head when the same passes over the servo signal written on the tracks. By subjecting this output signal to arithmetic operation, a position signal indicating the current position of the magnetic head is obtained. When the magnetic head is situated, in its entirety, on one track, the output signal is "0". When the magnetic head is radially displaced from a position corresponding to said one track on which it is currently located, the output signal becomes positive or negative. When the magnetic head is located at a middle position between said one track and a track adjacent thereto, the output signal obtains a positive or negative maximal value. And when the magnetic head comes near an adjacent track, the position signal approaches to "0". Each time the magnetic head passes over the servo sector, the servo device checks the output signal and, by confirming that the output signal varies from a positive value to a negative value or vice versa, detects that the magnetic head radially crosses a track.
In the prior art, therefore, when the magnetic head radially moves over a zone of more than a onetrack width during its passing over of the data sector, the output signal often becomes positive from its positive level or negative from its negative level, it being thus no longer detected that the magnetic head has crossed a track. For this reason, it was necessary to limit the movement of the magnetic head so as to prevent the same from moving over more than a one-track width zone during its passing over of the data sector. This means imposing an upper limit upon the speed with which the magnetic head moves, thereby restricting any increase in operating speed of the magnetic disk apparatus.